Little is known about the validity of DSM-IV criteria for Alcohol Use Disorders (AUDs) when applied to adolescents, and the natural history of adolescent-onset AUDs and comorbid psychopathology. This application proposes a program of longitudinal research on the nosology, comorbidity and course of AUDs through adolescence and into young adulthood. We will prospectively study 504 adolescents from addiction treatment programs, and a representative community sample of 206 adolescents. All subjects already have participated in an extensive baseline assessment of lifetime and current AUDs, other Substance Use Disorders (SUDs) and comorbid psychopathology, alcohol and drug use history, and other measures of functioning and problem severity. The proposed research will complete 1- year, 3-year and 5-year follow-up assessments of these subjects, using the same measures as the baseline assessment as well as other measures of young adult psychological and psychosocial functioning. Longitudinal data will be used to address three specific aims: 1) to develop and refine a new developmental model of diagnostic criteria for adolescent AUDs, and contrast its concurrent and predictive validity with DSM-IV criteria; 2) to characterize pathways and predictors of the course of AUDs through adolescence and into young adulthood; and 3) to determine the effects of comorbid psychopathology on the developmental trajectories of adolescent-onset AUDs. This research will incorporate the broader context of polydrug use and other SUDs in which adolescent-onset AUDs are usually embedded. The extensive data already collected on a large clinical sample and a community comparison group make this a highly feasible longitudinal project. The results will provide important new information about the natural history of AUDs and comorbid conditions across the developmental period of adolescence, and their impact on young adult alcohol and substance disorders and psychosocial functioning.